Recently, with the generalization of information communication services, the introduction of various multimedia services, and the advent of high-quality services, a demand on wireless communication services has been rapidly increased. Various multiple antenna systems are proposed to provide high-speed, high-quality data services in wireless communication. In the multiple antenna system, a transmitter and a receiver use multiple antennas. The multiple antenna system is also referred to as a multiple input multiple output (MIMO) system.
One example of the MIMO system is a multi-user (MU) MIMO system for allowing multiple users to effectively use spatial resources. Many techniques have been proposed to increase channel capacity in the MU MIMO system, and one of them is a dirty paper coding (DPC) scheme. According to the DPC scheme, data of other users is removed in advance by a base station (BS) when the data acts as interference to each user, and thus interference from other users is reduced. Therefore, the MIMO system can provide maximum channel capacity. However, the DPC scheme has a problem in that it is difficult to be implemented in an actual MIMO system since not only a transmitting end requires much channel information but also computational complexity is high.
One example of a method for increasing channel capacity of the MU MIMO system is a per user unitary and rate control (PU2RC) scheme in which spatial resources are allocated simultaneously to different users. The PU2RC scheme is proposed as a contribution document entitled “Downlink MIMO for EUTRA” of 3GPP TSG RAN WG1 #44/R1-060335 by Samsung Electronics Ltd. According to this scheme, each user selects a precoding matrix capable of maximizing a channel transfer rate of each user from a plurality of precoding matrices having an orthogonal basis, and feeds back a signal to interference plus noise ratio (SINR) and an index of the selected precoding matrix to a BS. On the basis of information fed back from each user, the BS determines and transmits a user and a precoding matrix for maximizing a total data rate.
The PU2RC scheme uses a spatial multi-user diversity to increase system capacity, and has an advantage in that it can be implemented in an actual MIMO system due to low computational complexity. However, the PU2RC scheme determines the user and the precoding matrix by considering only a single-cell environment and does not consider a multi-cell environment where inter-cell interference occurs. In particular, since a user located in a cell edge experiences a lot of interferences from neighboring cells, it is difficult to ensure a data rate of a specific level or higher when inter-cell interference is not considered.
Several schemes have currently been discussed to solve the problem of inter-cell interference occurring in the multi-cell environment. One example of these schemes is a BS cooperation scheme. A MIMO operation technique based on cooperation between BSs has recently been proposed in many papers. For example, a paper entitled “Base Station Cooperation for Multiuser MIMO: Joint Transmission and BS Selection” by HongyuanZhang, HuaiyuDai, and QuanZhou describes a DPC scheme based on BS cooperation or joint zero forcing beam forming (ZF-BF) scheme. According to this paper, all BSs have to perfectly know channel information in order to apply the DPC scheme based on BS cooperation or the joint ZF-BF scheme. Therefore, it is almost impossible to implement the BS cooperation scheme in an actual system due to feedback or complexity.
The BS cooperation scheme is also discussed in a paper entitled “Receiver-Enhanced Cooperative Spatial Multiplexing with Hybrid Channel Knowledge” by Hilde Skjevling, David Gesbert, and Are HjØrungnes. In this paper, a method for BS cooperation is proposed under the assumption that channel information of a neighboring cell uses only statistic information, and channel information of a cell for receiving a service is perfectly known. However, since the channel information of the cell for receiving the service of the BS is assumed to be perfectly known, it is also difficult to apply the scheme proposed in this paper to an actual system.